Page 442 - Instrumentation Reference Book 3E

I measurement

M. L. SANDERSON

nits and standards of tromotive force of 1 V is produced when the elec-
electrical measurement tric current varies uniformly at the rate of I AIS.
The weber (Wb), the unit of magnetic flux, is
20.1.1 SI electrical units the flux which linking a circuit of one turn would
produce in it an electromotive force of 1 V if it
The ampere (A) is the SI base unit (Goldman and were reduced to zero at a uniform rate in 1 s.
Bell 1982: Bailey 1982). The 9th General Confer- The tesla (T) is a flux density of 1 Wb/m2.
ence of Weights and Measures (CGPM) in 1948
adopted the definition of the ampere as that con-
stani current which, if maintained in two straight 20.1.2 Realization of the SI base unit
parallel conductors of infinite length. of negligi- The definition of the SI ampere does not provide
ble circular cross section. and placed 1 m apart in a suitable -‘recipe” for its physical realization.
vacuum, would produce between these conduc- The realization of the ampere has thus tradition-
tors a force equal to 2 x newton per meter of ally been undertaken by means of the Ayrton-
length. The forcehit length, FII, between two Jones current balance (Vigoureux 1965, 1971).
such conductors separated by a distance d when The force, Fy, in a given direction between two
each is carrying a current I A is given by: electrical circuits carrying the same current I is
given by
dM
F -12.-
x-
dx
where p() is the permeability of free space. Thus where M is the mutual inductance between them.
inherent in this definition of the ampere is the In the current balance the force between cur-
value of po as exactly 477 x 10-7N/A2. rent-carrying coils is weighed against standard
The derived SI electrical units are defined as masses. The principle of the balance is shown in
follows. Figure 20.1. The balance has two suspended coils
The volt (V), the unit of potential difference and and two pairs of fixed coils through which the
electromotive force, is the potential difference same current flows. If the upper and lower coils
between two points of a conducting wire carrying of the fixed pair carry current in the same direc-
a constant current of 1 A, when the power dissi- tion then the suspended coil eyaeriences no force.
pated between these points is equal to 1 W.
The ohni (n), the unit of electrical resistance, is
the electric resistance between two points of a
conductor when a constant potential difference
of IV, applied to these points. produces in the
conductor a current of 1 A, the conductor not
being the seat of any electromotive force.
The coulomb (C), the unit of quantity of elec-
tricity, is the quantity of electricity carried in 1 s
by a current of 1 A.
The farad (F), the unit of capacitance, is the
capacitance of a capacitor between the plates of
which there appears a potential difference of 1 V
when it is charged by a quantity of electricity of Fixed-
1 c.
The henry (H), the unit of electric inductance, is 1
the inductance of a closed circuit in which an elec- Figure 20.1 The current balance.